Events during July
The Earth is at aphelion, its farthest distance from the Sun (152 million kilometres) on the 7th.
Moon: Last quarter – 7th, New moon – 14th, First quarter – 22nd, Full – 30th.
This month┤s snapshots, at two hourly intervals between sunset and sunrise, show the progress of the planets, rising in the east
and setting in the west, on the night of the 15th/16th July 2007. They demonstrate how the planets all follow a similar path through the thirteen
(including Ophiuchus) constellations that make up the zodiac. The apparent path of the Sun┤s motion, as seen from Earth, is called the ecliptic,
which is shown on the snapshots as a curved orange line. The planets in our Solar System orbit the Sun in the same plane, known as the ecliptic
plane, with just a few degrees difference. At 7║, Mercury has the greatest orbital inclination of the planets. Pluto has 17║ but is no longer
classified as a planet. The Moon is inclined by about 5║ to the ecliptic, so also traces a similar path. There is no point in trying to identify an
object as a planet in any other area of the sky, as they simply won┤t be there. Knowing the path that the planets travel in makes them a lot easier
to spot and zoom in on.
To run through the snapshots:
21.00h – at sunset the day old Moon is low in the western sky and will shortly also set, followed by Saturn and Venus. Jupiter
can be seen in the southeast.
23.00h – the Moon and Saturn have disappeared below the horizon and Venus is about to. Neptune has just risen in the east.
01.00h – Jupiter and Neptune are progressing westwards across the sky and Uranus has risen.
03.00h – Mars has just risen.
05.00h – Jupiter has now set.
07.00h – sunrise and Mercury has risen just before the Sun.
The snapshots also show the apparent movement of the familiar constellation Ursa Major (containing The Plough) and the north star
Polaris, both of which can be seen throughout the night. Because Polaris lies almost in a direct line with the axis of the Earth┤s rotation above
the North Pole, it is almost motionless in the sky with the stars of the northern sky rotating around it. In the snapshots the bottom of the letter
“P” points to Polaris.
||Mercury, in the morning sky, is at greatest western elongation of 20║ on the 20th when it rises 1.5
hours before the Sun. The Moon is above on the 13th.
The best times to observe Mercury are when it is an evening star in the spring and a morning star in the autumn. In midsummer the lighter skies make visibility difficult near the horizon.
||Venus moves into Leo, setting 2 hours after the Sun on the 1st but just minutes after by the end of
July. It is 1║ below Saturn on the 1st and has the Moon nearby on the 17th.
On the 8th June 2004, Venus was at inferior conjunction and transited the sun. Transits of Venus are rare, taking place at greater
than 100 year intervals and usually in pairs. The last two transits of Venus were in 1874 and 1882. June's transit
began at 7.20h and lasted 6 hours until 13.20h, the total event visible from Europe as a small black disc crossing
the lower part of the sun from left to right. The next transit will be in late June 2012. After that, transits of Venus
won't occur again until 2117 and 2125.
Before and after inferior conjuction, when Venus is
the closest it comes to the earth, are the times at which the planet is most brilliant and can be seen setting or rising
4 hours after or before the sun. The dates of the next two inferior conjunctions are October 28th 2010 and October 26th 2018.
||Mars moves from Aries into Taurus and rises before 02.00h by the end of July. The Moon is above on the 9th.
At opposition on the 28th August 2003, Mars was only 56 million kilometres from the earth. It showed a
disc of 25.1 seconds of arc across which is almost as large as it can ever appear. Mars started 2003 at 310 million kilometres from
the earth at 4.5 seconds of arc and 1.6 magnitude. By opposition it brightened 50 times to reach -2.9 magnitude but faded to 0
magnitude by December. Even to the naked eye Mars was a striking object in the summer and autumn sky, easily identifiable by its
reddish hue in an area of sky poor in bright stars. Mars will not be as close again for another 15 years.
These favourable oppositions occur every 15 years but other oppositions occur at average intervals of 2 years 2 months. In general Mars is observable every other year, being too close to the sun for favourable conditions during other times. Brightness at opposition varies from -1.0 to -2.9 magnitude, and when furthest from the earth it fades to 1.7 magnitude. The planet can be identified by its orange-red colour.
||Jupiter is in Ophiuchus at –2.4 magnitude, setting soon after 02.00h by the 31st. The Moon is below on
Being 770 million kilometres from the sun, the difference in brightness between opposition and conjunction varies less than with Mars, from about -2.8 to -1.8 magnitude.
The 4 largest moons of Jupiter are easily visible through a small telescope, ranging from 4.6 to 5.6 in magnitude. The innermost, Io, takes 1.8 days to orbit the planet making its motion easily detectable within a few minutes.
||Saturn remains in Leo, setting minutes after the Sun by the 31st. The Moon is nearby on the 16th.
Saturn crossed the equator into the northern hemisphere in 1996 where it will remain until 2010 with the southern side of the ring system facing the earth. Because of its distance, its brightness varies little between opposition and conjunction but is affected by the huge ring system. Seen edge on the rings contribute little or no light.
Every 15 years the plane of Saturn's rings passes through the sun, illuminating first the north and then the south side. For a few days the rings are edge on to the sun. About the same time the earth passes through the ring plane and, depending on the earth's position this may happen just once or 3 times. During 1995/96 there was a triple crossing and the next will be 2038/39. The next single crossings will be in 2009 and 2025.
Saturn's largest moon, Titan, is visible in small telescopes orbiting outside of the ring system.
||Uranus, in Aquarius, rises about 23.00h by the end of July. The Moon is nearby on the 5th.
Brightness varies slightly reaching 5.6 magnitude at opposition. This is bright enough to see with the naked eye but identifying it against the stars is difficult.
||Neptune, in Capricorn, rises soon after sunset by the 31st. The Moon is nearby on the 3rd and 30th.
Neptune has an average magnitude of 7.9 which varies little with changing distance.
||No longer an offical planet and never brighter than 13 magnitude, Pluto is only visible through powerful
telescopes and we therefore do not report on its position in the sky.
Full moon: 7th
Last quarter: 14th
New moon: 22nd
First quarter: 30th
The Moon in 2007:
The Moon was seen unusually high and low in the sky during 2006 (see Moon). Although the extremities occurred last year, the Moon will still be seen very
high and very low each month this year, most noticeable around full Moon. It will be near full and high around the 2nd and 29th January, the 25th February
and the 24th March; and near full and low around the 1st and 28th June.
During each month of 2007, the Moon will pass through the Pleiades. This will mostly be in daylight but observable 01.00-04.00h 7th August,
0.00-03.00h 28th October and 22.00-01.00h on the 21st to 22nd December. Binoculars will give a clearer view.